Flexible thermoplastic films are used in a variety of applications including the construction of packaging and containers, protective films and coatings, and even wall paper. Typical thermoplastic polymers types include polyethylene (PE), polyethylene terephthalate (PET), and polypropylene (PP). In turn, PP can be found in different grades such as homopolymer, random copolymer, and impact copolymer. Films can be blown or cast, and subsequently are typically stretched. Stretching can be in the machine direction, across the machine direction (i.e., traverse direction), or biaxially stretched Films may have one or more layers (of the same or different polymer types).
There is generally a need to provide visual aesthetics to PP films so products or packaging is more attractive to consumers or connotes higher quality. Examples of desirable aesthetic effects include pearlescent, metallic-like visual effects, increased opacity, and combinations thereof. Conventional approaches to providing these aesthetic effects to films include the use of metallic or pearlescent agents, or metallic or pearlescent inks. However, these ingredients are generally expensive and thus are cost prohibitive in many applications.
One way to characterize these pearlescent and/or metallic-like aesthetic effects from films is by way of a Flop Index. Briefly, Flop Index is the measurement on the change in reflectance of a color as it is rotated through the range of viewing angles. A Flop Index of 0 indicates a solid color, while a very high metallic or pearlescent color may have a Flop Index of 15. There is a need to provide PP films that have desirable aesthetic effects without, or at least minimizing, the use of expensive pearlescent/metallic agents or pearlescent/metallic inks, while preferably being cost effective.
One example of desirable aesthetic effects is opacity. In some applications, film opacity connotes quality. One conventional way of providing opacity to films is the use of opacifiers such as titanium dioxide. However, there are potential drawbacks to using titanium dioxide. The ingredient is generally expensive for many applications. Moreover, it has been reported that higher levels of titanium dioxide in some films may reduce sealing performance in subsequent forming or packing processes. Furthermore, high titanium dioxide loading levels tend to have titanium dioxide distribution problem in some films, in which the titanium dioxide particles form a gel in the film and cause a so called “fish eye” defect in the film. Yet further, this defect may bring in further defects in printing thereby harming the overall aesthetics of printed film. One way to characterize opacity is by ISO method 6504. There is a need to provide PP films that have improved opacity without, or at least minimizing, the use of opacifiers (such as titanium dioxide), while preferably exhibiting desired film aesthetic effects (and doing so cost effectively).
Applicant has discovered a PP film exhibiting desirable aesthetics effects by blending a PP and a relatively high level of silicone in a film formulation, where the silicone and PP in the subject film layer are stretched at a relatively low stretching temperature (e.g. far below 70° C.). However, low stretching temperature may pose difficulties in certain large-scale manufacturing process (e.g. slow speed, limited stretch ratios, or film failures during process).
Therefore, there is a need to provide a PP film that exhibits desirable aesthetics effects, while preferably eliminates, or at least minimizes, the use of expensive and/or performance inhibiting ingredients, and can also be processed at moderate or even higher temperature processing conditions that is typically characteristic of large scale production.